1. Field of the Invention
The present invention is directed to methods and apparatus for detecting and locating signals. More specifically, the present invention is directed to methods and apparatus for detecting and locating signals with a three dimensional receiving element having sensors surrounding its surface.
2. Description of the Related Art
Overall Methods to Locate EM in 3D
Current methods to locate electromagnetic waves in three dimensions rely on intensity, wavelength and phase measurements using planar sensor arrays combined with signal and image processing algorithms. In lower frequency systems, measurements taken by planar sensor arrays are correlated to find the 3D location of electromagnetic wave sources. By measuring the phase shift of waves between sensors, the position of the source can be triangulated. Higher frequency systems in the visible light and infrared range typically use imaging system to determine 3D location of sources. Other systems for 3D location use active EM beams with sensors that measure the reflected waves. This type of system is currently used in LIDAR and radar systems for a vast number of applications.
Radio Frequency Systems
Current methods to locate radio frequency waves rely on a form of triangulation, whether a single directional antennae system or a phased array radar system with multiple antennas and signal processing algorithms. A single antenna typically monitors signal amplitude to find the direction of the desired radio frequency source. This method involves user-operated search patterns susceptible to human error and lengthy search times in emergency or time-critical scenarios. Back-country travelers encountering potentially dangerous avalanche terrain can use radio frequency transceivers in the case of possible burial. These transceivers determine a direction to buried avalanche victims using traditional triangulation techniques. The method to locate buried victims involves a search pattern that increases the time for rescue. Locating buried transmitters involves the use of antennae placed perpendicular to one another and then processing the phase shift of the incoming signals. The most recent embodiment of this application is shown in U.S. Pat. No. 5,955,982. “The first antenna, the second antenna, and the third virtual antenna provide three dimensional vector analysis by the receiver of the predetermined frequency received from the radio transmitter.” This method uses a predetermined frequency and monitors phase shift between two antennae to triangulate the signal source. The method susceptible to multi-path errors associated with reflecting waves from the surrounding environment.
In addition to single antennae systems, radar systems employ numerous antennas to monitor values such as wavelength, amplitude, and phase to determine the location of RF reflecting objects. These systems emit sweeps of microwaves on the surrounding area and capture returning signals. Such sweeps typically encounter various mediums that can reflect microwaves. Current systems must subsequently compensate for this multi-path reflection with advanced signal processing algorithms.
Electro-Optic and Infrared Systems
The current state of the art for sensing infrared sources in 3D employs imaging systems to take snapshots of the surrounding area. These imaging systems are limited to a specific field of view (FOV) and incorporate scanning algorithms and image processing techniques for target tracking and identification. The instantaneous FOV of the imaging systems can only observe a small portion of the 3D space at any one time. The drawbacks of conventional imaging systems are detailed in The Handbook of Infrared Technology, “with a limited field-of-view system, a large distance is required to view a large area. But then the target becomes very small and may be hard to find. Shorter distances mean that a smaller area is viewed and overall search time is significantly increased.” These limitations on contemporary imaging systems cause significant difficulties when monitoring a wide field of regard for infrared sources.
There exists a wide range of systems for detecting and deterring infrared seeking threats. The current state of the art technologies include the LAIRCOM and NEMESIS systems designed by Northrop Grumman, and the DART system by BAE. As discussed on a Northrop Grumman brochure, “the LAIRCOM system uses staring missile warners to detect a launched MANPAD then directs a pointer-tracker, which locks on to the missile in flight, and jams the missile's guidance system with a beam of infrared energy.” The missile warners have a wide FOV and are designed for long range detection. The system uses up to six, 120° field of view sensors to view an entire 360° field of regard (FOR). The LAIRCOM system first scans the six missile warners to detect ultra violet UV radiation emanating from missile threats. The missile warners take images of the 120° FOV and then provide guidance to the gimbaled system with a narrow FOV. Using the narrow FOV, the gimbaled system precisely tracks the target for IR beam directing. The beam of infrared energy saturates the infrared sensors on the incoming missile and disables the guidance system. The NEMESIS and DART systems use similar techniques to the LAIRCOM to defeat incoming infrared seeking missiles.
The missile warners are the first components of the system to detect missile threats and can be analyzed to determine the initial time constraints. Some considerations when determining their effectiveness include the wide FOV, lens focus, image processing algorithms, and the need for multiple sensors. The wide 120° FOV samples a very large area for UV sources, therefore a small UV source will become very small in the image plane. In addition, sources which are near the 120° range of the system become obscured on the image plane and must be corrected with image processing algorithms. Complex image processing algorithms must be incorporated to determine the 3D direction of incoming threats. Lensing systems are also subject to a system focus, objects outside the focus of the system will be obscured. A finite time will be necessary to focus the lens system on the object. The current systems use up to six different missile warning systems which must all be analyzed using image processing techniques. The combination of wide FOV, focus time, image processing algorithms, and multiple sensors creates a complex, high-cost system with many components to determine the 3D location of an IR source.
Once the IR source is detected by the missile warning system, the gimbaled system points a narrow FOV imaging system and beam of infrared energy at the incoming MANPAD. The DART system currently has start and stop slew rates of 0.25 seconds, which will amount to at least 0.5 seconds before the target can be locked onto. Additional time is necessary to travel to the angular coordinates of the incoming missile. With a 90 degree per second slew rate, it would take from 0 to 3 seconds to move to various angular coordinates.
Other systems utilize multiple FOV sensors for searching and tracking. These systems use a wide FOV in order to search for IR threats and then a narrow FOV for tracking. Examples of these systems include the Night Hunter II and the RAVEN EYE II by Northrop Grumman, and the POP by IAI. The actuator speed varies depending on the specifications of the system. The Raven Eye II has actuator speeds of up to 60 degrees/sec and a wide FOV of 13.5×13.5 for searching. With these specifications it would take up to 78 seconds to monitor a full 360 degree field of regard for IR sources.
LIDAR Systems
Light Detection and Ranging (LIDAR) systems, or laser radar, function by sending out pulses of light and processing the returning signals. By measuring the time of the photon flight, LIDAR systems spatially derive the surrounding environment. Differential Absorption LIDAR (DIAL) employs similar techniques of LIDAR but also includes a pulse of two discrete wavelengths to identify and measure the concentration of certain gases. Different gases (such as ozone and water vapor) absorb and transmit different wavelengths within the EM spectrum. The relative strength of the returned wavelengths as measured by the detector, determine the specific gas. Gaseous targets are notorious for scattering incoming beams and discouraging accurate analysis. Planar techniques used to acquire the returned signals are prone to multi-path errors resulting from scattering. Unintended reflecting objects will interfere with the analysis of intended objects. These systems are used for a wide range of applications including laser guided missile systems, weather monitoring, toxic cloud analysis, vegetation analysis.
A need remains in the art for improved methods and apparatus for detecting and locating signals.